1. Field of the Invention
The invention relates to an avalanche photodiode and, particularly, to an avalanche photodiode for optical communication.
2. Description of the Related Art
As avalanche multiplication type semiconductor light-receiving elements for optical communication, elements using InGaAs lattice-matching with InP in the light-absorbing layer are widely used (see, for example, Hirho Yonetsu, “Optical Commununication Element Engineering” Kogaku Tosho (K.K.)). In this avalanche photodiode (APD), either carrier of an electron-hole pair generated in the light-absorbing layer is accelerated by an electric field and injected into the multiplication layer. A high electric field is applied to the multiplication layer, where these injected electrons or holes are accelerated more and eventually ionized. The characteristics of the APD allows its important noise characteristics to be decided by the ionizing process of carriers in this multiplication process.
Specifically, if the ratio (ratio of ionization rates) of the electrons to holes ionized in this multiplication layer is increased, the noise characteristics are expected to be improved. Here, although the ionization rate (α) of the electrons may be larger than the ionization rate (β) of the holes and vice versa, the ratio (α/β or β/α) of ionization rates is preferably larger. Also, with regard to high-speed response characteristics which are important characteristics similarly to the noise characteristics, the ratio of ionization rates has a large influence thereon. The high-speed response characteristics are decided by the time constant defined by element resistance and element capacity, the time required for carriers to pass through the absorption layer and multiplication layer and the ratio of ionization rates. To tell the relation to the ratio of ionization rates, a larger ratio of ionization rates brings about better high-speed response characteristics.
However, the ratio of ionization rates is decided depending on the type of material When InP is used in the multiplication layer, the ionization rate of holes is large. However, the ratio (β/α) of the ionization rates is about 2 at most. Therefore, APDs using AlInAs having a high ratio of ionization rates in the multiplication layer are being developed.
If this AlInAs is used in the multiplication layer, the ionization rate of electrons can be increased (α>β) and an ionization rate of about 4 is obtained. Besides the above, researches and developments are made intensively to increase the ionization rate by making the multiplication layer have an AlInAs super lattice structure as reported in an article entitled “Planer Type Super Lattice APD” which is Isao Watanabe & 5 others, “Planer Type Super Lattice APD”, Shingaku Giho, LQE97-79, 1997-10, pp 69-74.
However, when the multiplication in layers other than the multiplication layer is caused even if the ratio of ionization rates of the multiplication layer is improved, the ratio of ionization rates drops, causing deteriorations in noise characteristics and high-speed response characteristics. Particularly, in semiconductors having a narrow bandgap, multiplication is caused easily in a lower electric field. Therefore, multiplication takes place very easily in the case of InGaAs used for the light-absorbing layer in APDs for optical communication When multiplication takes place in InGaAs whose ratio (α/β)of ionization rates is about 2, the ratio of ionization rates drops in APDs using InP which multiplies (β>α) holes for the multiplication layer.
Also, in the case of APDs using AlInAs which multiplies (α>β) electrons as in the case of InGaAs for the multiplication layer, the ratio of ionization rates drops because the ratio of ionization rates of AlInAs is about 4. For this, usual APDs are used in the situation where a high electric field is applied to the InGaAs absorbing layer, giving rise to the problem that noise characteristics are impaired. Moreover, supposing that a constant electric field is applied to the InGaAs absorbing layer, the multiplied carriers are increased and the ratio of ionization rates drops in the case of making the InGaAs absorbing layer thick. Therefore, in current APDs for communication, there is a tradeoff relation between sensitivity characteristics and noise characteristics and it is therefore difficult to attain the compatibility between sensitivity characteristics and noise characteristics.
Also, in high-speed response APDs, the running time of carriers generated in the depletion layer and the diffusion time of carriers generated in the non-depletion region largely affect high-speed response characteristics. As measures taken for this, there are, for example, methods in which the carrier density in an absorbing layer which is undepleted is changed to thereby control an internal electric field and the over-shooting speed of electrons are utilized to thereby shorten the diffusion time of carries as reported in Ning, Li, et. al. “InGaAs/InAlAs avalanche photo diode with undepleted absorber”, Appl. Phys. Lett: 31, Mar. 2003).
However, because, in the above structure, impurities are added to the InGaAs absorbing layer region to form the non-depletion region, it is necessary to make thin the absorbing layer to obtain an electric field higher enough to cause the over-shooting of electrons in the whole absorbing layer region. However, if the absorbing layer is made thin, the light to be transmitted is increased, posing the problem that the sensitivity which is important characteristics for APDs is deteriorated.
Also, a Publication of JP-A No. 2000-22197 discloses an avalanche photodiode in which the semiconductor light-absorbing layer is constituted of two layers consisting of a depletion region which is adjacent to the semiconductor field limiting layer and has a thickness of 10 nm or more and 0.3 μm or less and a non-depletion region which is also adjacent to the semiconductor field limiting layer and has a thickness of 2 μm or less.
However, when light is incident from the upper surface of the element in usual, the incident light is almost absorbed in the semiconductor absorbing layer which is to be undepleted because the light-absorbing layer to be depleted is thin. In this case, the electron-hole pairs generated in the absorbing layer almost constitute diffusion current and therefore, it takes considerable time to draw the diffusion current as signals, leading to a deterioration in high-speed response characteristics. Also, these electron-hole pairs are recombined in the non-depletion region before the diffusion current is drawn as signals, posing the problem that the diffused current is not drawn as signals, causing a deterioration in receiving sensitivity.